The most common vulcanizing agents widely used for unsaturated rubbers, such as, e .g., natural rubber (NR), isoprene rubber (IR), butadiene rubber (BR) and styrene/butadiene rubber (SBR) is sulfur in elemental form. For the production of rubber, approx. 0.25 to 5.0 parts by wt. sulfur, based on 100 parts by wt. rubber, are used. The amount of sulfur, which is effectively used, depends on the amount of vulcanization accelerator chosen, which in the end is determined be the desired properties of the vulcanization products.
Vulcanizing systems which are very often used are the conventional and the semi-efficient vulcanizing system. The conventional vulcanizing system has a high sulfur content and low vulcanization accelerator content, while the semi-efficient vulcanizing system has a moderate dosage of sulfur and vulcanizates accelerator. The typical dosages are known to the expert. They are described e.g. in W. Hofmann, Kautschuk-Technologie, Genter Verlag, Stuttgart, 1980 p. 64 and 254-255. Conventional vulcanizing systems result in vulcanizates with good resistance to dynamic stresses (flex life), but these are very sensitive to aging and reversion. Semi-efficient vulcanizing systems usually give vulcanizates which have a less of a resistance to dynamic stresses (flex life), but, in return, they are somewhat more stable to aging and reversion (cf. R. N. Datta and W. F. Helt, Rubber World, August 1997, p. 24, et seq.)
Reversion is understood as meaning a rearrangement of network bridges which takes place under the action of heat in the absence of oxygen and leads to a deterioration in the use properties of the vulcanizate and is thus, undesirable (anaerobic aging). Reversion is unavoidable during the vulcanization of very thick and voluminous components, e.g. in truck and buis tires and fenders. The physical explanation of this is simple: when the inner volumes of rubber mixture are just correctly vulcanized by the heat flow introduced via the hot vulcanization mould, the rubber mixture volumes directly adjacent to the hot vulcanization mold are, of course, already over-vulcanized On the other hand, during use of these rubber components under long lasting, intensive dynamic stress, an increase in temperature (heat build-up) of the rubber component takes place due to hysteresis (cf. flexing, of the tire), which causes reversion of the vulcanizates. Reversion can take place to such an extent that it even leads to destruction and therefore, unusability of the vulcanizate.
Some reversion stabilizer speciality products have been disclosed in recent years which either minimize reversion by incorporation of heat-stable network bridges which are practically incapable of reversion (cf. EP-A 530 590), or which replace the broken conventional network crosslinks after reversion has already taken place by other more stable crosslinks (cf. R. N. Datta and W. F. Helt, Rubber World, August 1997, p. 24 et seq.).
Commercially available reversion stabilizer speciality products are the disodium salt of hexamethylene-1,6-dithiosulfate dihydrate and 1,3-bis(citraconimidomethyl)-benzene.
A general disadvantage of these commercial reversion stabilizer speciality products is their relatively high price, which is caused by educts which are not available in large amounts and by the difficult and expensive preparation of these products, as a result of which widespread use is prevented in the rubber-processing industry, which is under constant pressure to reduce costs, in particular in the tire industry. A specific disadvantage of disodium hexamethylene-1,6-dithiosulfite dihydrate is its expensive delivery form. Because of its salt character, it must be ground very finely in view of good mixing-in properties, which on the other hand from the industrial hygiene aspect again involves oiling the powder to suppress dust.
A specific disadvantage of 1,3-bis(citraconimidomethyl)-benzene is that it can become active in the vulcanizate solely and only if reversion has already started in the unsaturated rubber crosslinked with sulfur and conjugated olefins have thus formed, which in turn can react in a post-crosslinking with the citraconic derivatives (via Diels-Alder reaction) to give a new but now different network.
A disadvantage of the vulcanizing agents of EP-A 530 590 which have been described is their high molecular weight, compared with the species actually having the crosslinking action.
A large number of reaction products of olefins and sulfur have been disclosed in the past: EP-A 258 168 thus discloses the reaction of olefins with sulfur in water, it being possible for bases to be present as catalysts. Styrene, alpha-methylstyrene, cyclopentadiene and dicyclopentadiene are mentioned as preferred olefins (page 4, lines 14 and 15). Cyclopentadiene de facto does not occur in the embodiment examples according to EP-A 258 168. On page 7, lines 57-58 and page 8, lines 1-3, on the subject of industrial applicability it is disclosed that the crosslinking agents according to EP-A 258 168 lead to vulcanizates which are comparable in their physical properties to the properties obtained with a conventional sulfur crosslinking system. An improved stability of the vulcanizates according to EP-A 258 168 to reversion has not been disclosed and is also not described. Our own experiments (cf. Example 1) have shown that the products according to EP-A 258 168 of dicyclopentadiene and sulfur give vulcanizates which do not reveal an improved resistance to reversion. The particular advantage of the crosslinking agents according to EP-A 258 168, however, lies in the fact that, compared with sulfur, they do not bloom or bloom less. The structures of the vulcanizing agents according to EP-A 258 168 are described (cf. H. Colvin and Ch. Bull, Gummi, Fasern, Kunststoffe 8 (1997) 627-634 and Rubber Chemistry & Technology vol. 68 issue 5, November-December 1995, p. 746-756). They are polymers of sulfur and polycyclic hydrocarbons. These compounds are completely different in terms of structure from the compounds of the present invention.
On the basis of EP-A 258 168, products of olefins and sulfur with particular particle diameters are described in WO-A 99/48966.
U.S. Pat. No. 3,259,598 describes the use of a product of sulfur, linseed oil and styrene as vulcanizing agents for rubber. U.S. Pat. No. 3,264,239, furthermore, discloses a vulcanizing agent of sulfur, linseed oil and dicyclopentadiene.
U.S. Pat. No. 3,523,926 discloses vulcanizing agents from diolefins, such as, e.g., cyclopentadiene and dicyclopentadiene, and sulfur with amines as a catalyst. At no point in this publication is the additional use of hydrogen sulfide described nor suggested.
U.S. Pat. No. 2,989,513 discloses polymers of sulfur and an olefin for the vulcanization of rubber. As useful olefins there are mentioned cyclopentene, inter alia, from the series of cycloalkenes in column 3, line 19, and cyclopenitadiene, inter alia, from the series of polyolefins in column 3, line 21. The reaction according to U.S. Pat. No. 2,989,513 is preferably carried out between 145.degree. and 160.degree. C. The embodiment examples include only copolymers of sulfur and styrene or of sulfur and ethylene or of sulfur and isobutylene. At no point in this publication is the additional use of hydrogen sulfide described nor suggested.
WO-A 94/12450 discloses a method for the preparation of sulfur-containing compounds from e.g. alkenes, such as e.g. hexadec-1-ene, dec-1-ene and octa-1,7-diene, and compounds of the formula MHS.sub.x, wherein M denotes a cation, such as e.g. Na.sup.+, K.sup.+ or NH.sub.4.sup.+. The compounds of the formula MHS.sub.x can be employed as vulcanizing agents for rubber, a solvent being employed.
It is also known to react cyclopentadiene with sulfanes to give di-cyclopentenylpolysulfanes.
The vulcanizates, which are conventionally widely produced from unsaturated rubbers are produced only with sulfur and an accelerator as the vulcanizing agent, i.e. that is to say without agents which prevent or reduce reversion. The properties of rubber vulcanizates produced conventionally with conventional and semi-efficient vulcanizing systems are in need of improvement. There is therefore a need for a vulcanizing agent for unsaturated rubbers which is predominantly based on synthesis units which are available in large amounts, is readily accessible and can therefore be prepared economically, and which can completely or partly replace the crystalline sulfur, which tends to bloom, in a vulcanizing system and leads to vulcanizates with an improved reversion stability and a lower beat build-up, in particular after over-curing.